In 1984, Winston and Julia are at Victory Square and watch a convoy of trucks passing. What is their primary cargo?

What Julia and Winston and the crowds packed into Victory Square see passing by in the trucks are prisoners of war captured by Oceania in a recent battle. As the novel puts it,

In the trucks little yellow men in shabby greenish uniforms were squatting, jammed close together. Their sad, Mongolian faces gazed out over the sides of the trucks utterly incurious. Occasionally when a truck jolted there was a clank-clank of metal: all the...

What Julia and Winston and the crowds packed into Victory Square see passing by in the trucks are prisoners of war captured by Oceania in a recent battle. As the novel puts it,

In the trucks little yellow men in shabby greenish uniforms were squatting, jammed close together. Their sad, Mongolian faces gazed out over the sides of the trucks utterly incurious. Occasionally when a truck jolted there was a clank-clank of metal: all the prisoners were wearing leg-irons...

As we know from the novel, Oceania is perpetually at war with one of the two other superpowers, Eurasia or Eastasia. Since both these other superpowers seem to be Asian, the ethnicity of the prisoners gives no clue as to what country Oceania is currently fighting with--and it doesn't really matter, because the country changes all the time, and whichever country it currently is, the regime insists that country has always been the enemy. But through much of the first part of the novel, Oceania is at war with Eurasia.

We can surmise that these prisoners will be executed because while rewriting a news story, Winston creates a Comrade Ogilvy who creates a hand grenade that kills 31 Eurasian prisoners in one burst.

In Part Two, Chapter One, Winston and Julia see a convoy of trucks pass through Victory Square. These trucks are carrying human cargo: that is, a group of Eurasian prisoners. This prompts a "din of shouting" and people to gather in crowds, eager to catch a glimpse of the prisoners.

Remember that for much of the story, Oceania is at war with Eurasia, hence the capture of these men. For the crowd, they are objects of hate but, for Winston, they are people to be pitied:

"Their sad, Mongolian faces gazed out of the side of the trucks utterly incurious."

These men are likely on their way to be executed as prisoners of war. But later, in Part Two and during Hate Week, Oceania will switch sides, declaring peace with Eurasia and war with Eastasia, and proving the Party's fickle nature.